In 2010, data from the Fermi Gamma-Ray Space Telescope revealed two huge gamma-ray-emitting bubbles extending 25,000 light-years in each direction from the Milky Way's center. Since this region of the galaxy is home both to a supermassive black hole and star formation activity, it was uncertain which of them produced the structures. A new analysis of radio and microwave observations has confirmed these bubbles exist—but found additional features suggestive of star formation, rather than black hole activity.

The new results looked at the polarization of the radio emission coming from the lobes. As reported by Ettore Carretti and colleagues, the polarized light revealed relatively strong magnetic fields. These are of a kind associated with rapid star formation, which is known to occur in the region 700 light-years around the galactic center. Additionally, the bubbles contain spiraling ridges that indicate fluctuations in the history of the star formation, which may be used to study the evolution of the Milky Way's nucleus over the last 10 million years.

Gamma rays and radio waves are on the opposite ends of the energy spectrum, but they are often produced by the same physical phenomena. However, they convey different information about their source. In particular, when electrons are accelerated by magnetic fields, they emit synchrotron radiation, radio light with a distinctive polarization signature—information not carried by gamma radiation.

The authors of the present study used data from the Parkes Radio Telescope in Australia and from the orbiting Wilkinson Microwave Anisotropy Probe (WMAP). All of these revealed lobes at the same location and of the same size as the Fermi gamma ray observations. Gamma ray, microwave, and radio emission from the same structure reveals the presence of a gas of electrons moving very rapidly, accelerated by magnetic fields. In this case, the data are consistent with electrons moving about 1000 km/s—fast enough to escape from the galaxy's gravitational pull. When background light collided with the energetic electrons, it was boosted to higher energies, explaining the gamma ray emission.

All three sets of data confirm that the bubbles are huge—each is about 1/4 of the diameter of the Milky Way—yet they originate in a tiny region at the center of the galaxy. In the absence of the radio and microwave data, astronomers and writers (including myself!) had speculated the lobes could be quasar jets from the Milky Way's central black hole. However, the distinctive magnetic field characteristics and ridge structures in the bubbles point to star formation as being the real culprit.

Intense star formation pumps a lot of matter into its surroundings, mostly in the form of electrons. It also results in strong magnetic fields, which carry energy outward. In the case of the Milky Way bubbles, the estimated magnetic fields radiate about 1032 Watts—a phenomenal amount of power by any standard, but consistent with the observed star formation rate within a 330 light-year radius of the galactic center.

The authors suggested the ridge features in the bubbles are the immediate output from the star formation. As these regions orbited the galactic center, their output spiraled, much like a lawn sprinkler. Over time, the entire bubble expanded, leaving the features seen by the Fermi observatory.

The researchers also proposed that the inflow of gas necessary to feed star formation may also be why the Milky Way's black hole is relatively quiet. Instead of landing on the black hole and producing jets, the gas is eaten up by new stars or blow out of the galaxy into the huge bubbles. The overall picture they present is consistent with the wealth of data from across the spectrum, and promises to clear up a number of questions.